Ductless Minisplit Heat Pumps

Heating and Cooling Without Central Ducts

Bird's eye view

With a ductless minisplit heating and cooling system, a single exterior compressor can be connected to a number of interior fan units. They are typically mounted on the wall, like this one, although some types can be recessed into the ceiling.

Ductless minisplits are specialized air-source heat pumps

Like conventional air-source heat pumps, ductless minisplits move heat from one place to another. In winter, they extract heat from outdoor air and move it inside. In summer, the process is reversed. The same equipment provides both heating and cooling. But unlike conventional equipment, these systems do not need a central network of ducts, simplifying installation and improving efficiency.

Although ductless minisplits are frequently promoted by manufacturers as an easy way of adding air conditioning to a house without ducts, they also can provide heat. Only a few interior units may be capable of heating and cooling a very well insulated house.

Key Materials

Still a small market share in U.S.

Standard air-source heat pumps are popular in moderate climates in the United States where cooling loads in the summer can be high but heating loads in winter are relatively modest. In the U.S., most air-source heat pump systems distribute heat through forced-air ductwork.

Ductless minisplit systems have a much smaller slice of the market than old-fashioned air-source heat pumps. The U.S. Department of Energy estimates that minisplits make up about 5% of the market for residential heating and cooling systems.

In Asian markets, however, ductless minisplit systems have been dominant for space conditioning, and some of their inherent advantages, especially for high-efficiency homes, may give them a boost here. In high-performance houses, they offer significant cost savings over more elaborate heating systems, such as ground-source heat pumps connected to a radiant-floor distribution system.

Design Notes

As many as eight interior fan units can be powered by a single outdoor compressor/condensor. This unit, at a net-zero ready house in Wells, Maine, is manufactured by Mitsubishi. It feeds four indoor fan units.

Zoned heating and cooling is easy

A single outdoor compressor/condenser can serve as many as eight indoor fan units, each in a separate room. Each indoor fan unit is controlled separately, either by a remote control or by a thermostat, so zoning heating and cooling is relatively simple. Zoned systems can save energy because conditioned air is provided only where it's needed.

Variable-speed fans and compressors help the system meet specific load requirements without cycling on and off as conventional systems do. This feature boosts efficiency.

Builder Tips

Adding air conditioning and heat without major renovations

Ductless minisplits are especially well suited to renovations because heating and cooling can be added without extensive interior renovations. Refrigerant is piped to indoor fan units through a conduit requiring only a 3-in. hole through the wall, and the outdoor unit can be located 50 ft. away from indoor fan units.

Indoor units are typically mounted on a wall, making them more obtrusive than centrally ducted heating and cooling equipment. Some manufacturers, however, also offer ceiling "cassettes" that can be flush-mounted in the ceiling. These are available for both commercial and residential applications.

The Code

Systems exceed efficiency requirements

The seasonal energy efficiency ratioSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. (or SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive.
) of some ductless minisplit equipment is 26, far above the federal minimum and Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. requirements.

OTHER CONSIDERATIONS

No 'commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency.
' with minisplits
Conventional ducted systems are prone to a variety of installation errors and oversights. As a result, forced-air systems are often "commissioned," which means they must be tested and adjusted so they perform properly.

In an interview with GBAGreenBuildingAdvisor.com's Martin Holladay, Marc Rosenbaum suggested that it might make more sense to use heating and cooling equipment that doesn't need commissioning.

"I can tell you what we haven't been doing: we haven't been commissioning our buildings with ductless minisplits," Rosenbaum told Holladay. "If the building has a system with a lot of zones and a central controller, there may be some training from the rep -- but no commissioning. These are self-contained products, so you are buying the control system with the unit. It's all packaged."

High-performance houses beckon
When a house has lots of insulation and is carefully air-sealed, it's possible to keep it comfortable with a very modest ductless system.

Carter Scott, a builder in Massachusetts, built a two-story, net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. house in 2008 that needed but two interior minisplit heads, one upstairs and one in the living room downstairs. As explained in a GBA blog on the topic (also by Martin Holladay), upstairs bedrooms stay comfortable even though the fan unit was located in a hallway.

"We have since built several houses in which the upstairs minisplit unit isn't even being used until the outdoor temperature drops below 20 degrees," Scott told Holladay.

The installed cost of the system was just over $5,000. This goes right to the heart of one argument green builders often make: investing in better insulation, more efficient windows and careful air-sealing allows steep cost reductions in mechanical systems.

GREEN POINTS

LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify.
Up to 4 points for very high efficiency HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment (Energy & Atmosphere).

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions.
Under Ch. 7 — Energy Efficiency: Up to 12 pts. based on system efficiency and climate (703.4.4).

ABOUT DUCTLESS MINISPLITS

Specialized heat pumps ideal for houses without ductwork

Ductless minisplit heat pumps are widely used in Asia but so far have a relatively small share of the market in the U.S., accounting for only about 5% of installations for heating and cooling systems. Given a number of key advantages over conventional air-source heat pumps, that could change.

Both systems work on the same principles. The big difference is in how conditioned air is distributed. With a conventional air-source heat pump, an outdoor condenser/compressor supplies a central indoor evaporator/air handler with pressurized refrigerant. Conditioned air is blown through ducts to individual rooms.

In a ductless system, the outdoor unit supplies as many as eight individual indoor units with refrigerant through a small-diameter line. There are no ducts. Each indoor unit can be controlled separately, making it relatively simple to manage heating and cooling in different parts of the house.

Ductless minisplits often are promoted primarily as an easy way of adding air conditioning to homes without ductwork.This is true, but they also can provide heat. They are an increasingly popular choice for high-efficiency houses with low heating loads.

Advantages of going ductless

Eliminating the need for ducts saves money, and a ductless system also is less disruptive during a renovation. In a ductless system, a 3-inch-diameter conduit provides enough room for the refrigerant piping, control wires and a drain for condensate. A simple system that includes an outdoor unit and a few indoor heads can be installed quickly, with little of the construction headaches associated with a conventional system.

Ducts also are inherently wasteful. The U.S. Department of Energy estimates that as much as 30% of the heating or cooling energy in a conventional duct system is wasted. If the ducts pass through an unconditioned attic or crawl space, or when ducts are not sealed very carefully, losses can be even greater.

Losses in a ductless installation are far lower. Fujitsu estimates that cooling losses in insulated refrigerant lines are less than 5%.

MORE ABOUT DUCTLESS MINISPLITS

Low temperature operation

The inability of most conventional air-source heat pumps to operate efficiently in below-freezing temperatures is one reason this heating and cooling option has mostly been limited to moderate climates. As temperatures fall into the 40s, supplemental heating is usually needed to keep indoor air temperatures comfortable. This might take the form of electrical resistance heat, or possibly heat from a gas-fired add-on furnace.

Many ductless minisplit models, however, continue to produce heat even when the outdoor temperature drops well below zero. Mitsubishi, for example, says its Hyper-Heat models work at 13 below zero and operate at 100% of capacity at temperatures as low as 5 degrees.

Unlike most conventional air-source heat pumps, ductless minisplits don't have any supplemental heating capacity built into them. So when temperatures do fall below the point where the unit can produce heat, homeowners will have to turn to a separate heating appliance -- a gas-fired appliance, electrical strip heaters, a wood stove, or something else.

Why do these devices operate so well at much lower temperatures than conventional air-source heat pumps? Mitsubishi attributes the ability it to inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements. technology that allows compressors to "ramp up quickly" to provide heat right away.

"Once the desired set point is reached, the system slows down to maintain the perfect temperature," the company says. "Traditional systems ramp up slowly at lower RPMs, overshoot the desired temperature and then turn of and off to maintain the desired set point. The higher operating speeds of the inverter-driven compressors also provide the ability to heat at lower outdoor temperatures."

While ductless minisplits can provide heat in very cold weather, efficiency declines as the outdoor temperature drops. The National Renewable Energy Laboratory says that measuring the efficiency of these systems is challenging, in part because of the variable-speed components designed to match output to demand more closely than an on-or-off heat pump.

"Most public information on MSHP [minisplit heat pump] performance is provided by equipment manufacturers, and is typically limited to performance at a single operating speed for heating and cooling," NREL said in the summary of a research paper on the topic.

Researchers developed testing strategies allowing them to measure, for the first time, performance of two ductless minisplit installations across a wide range of operating conditions.

"In the laboratory tests," the NREL report said, "researchers found that both MSHPs achieved manufacturer-reported performance at rating conditions. However, at other temperature and humidity conditions the heat pumps' capacity ranged from 40% above to 54% below the manufacturer-reported values. Knowing how performance varies is critical in order to reasonably estimate annual energy consumption of a MSHP, and to compare MSHPs to other heating and cooling options. Minisplit heat pump efficiency (COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415.
) was seen to significantly exceed rated efficiency at low compressor speeds -- a very important effect."

NREL's tests found that COPs (coefficient of performance) ranged from roughly 7 at an outdoor temperature of 55 degrees to a COP of 2 or less as outdoor temperatures fell to 5 below zero. COP is an important measure of system efficiency (although not the only benchmark used for heating and cooling equipment). It is the ratio of energy input to heating or cooling energy output.

Wall-mounted units are typical

Indoor units range in size and output, from 9,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules.
/h to 36,000 Btu/h, but they are often mounted on a wall close to the ceiling, and that makes them much more visible than other heat distribution systems. Floor registers for forced-air systems and baseboard radiators are much easier to camouflage. Radiant-floor heat distribution is, of course, completely out of sight.

This won't bother all homeowners, as a recent discussion in a Q&A post revealed. The writer was planning to heat and cool his 3300-sq. ft. house with a ductless minisplit system that incorporated seven indoor heads, each connected to its own outdoor compressor. The aesthetic of all those wall-mounted fan units wasn't a factor.

For those who object to the looks, there are other options, including units that are flush mounted into the ceiling. Fujitsu's ceiling-mounted cassettes, for example, show only a flat grille when installed. When installed in a retrofit, however, cutting into ceiling to install them would negate some of the low-impact advantages that ductless minisplits advertise in the first place.

Another consideration are the exterior service conduits that connect the outdoor compressor with interior fan units. That's not a factor in new construction, where lines can be buried inside walls, but the easiest way to install a system in an existing house is to run the line up the outside of the house, bore a hole through the wall and attach wiring and refrigeration lines to the indoor unit. And that's hard to hide.

Designers like the flexibility and cost

Ductless minisplits can be a less expensive heating and cooling option than conventional HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment, especially in a high-performance house that won't need many indoor heads because of reduced heating and cooling loads. A ductless system could be installed at a fraction of the cost, for example, of a ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. linked to a radiant-floor distribution system.

The National Association of Home Builders Research Center says the cost of installing ductless heat pumps has dropped as the technology becomes more established in the marketplace. A 2004 survey of suppliers found that costs ran between $500 and $900 per ton (a ton is equal to 12,000 Btu).

Operating costs, the Research Center added, also should be lower than for conventional heat pumps.

Cost can be advantage, but Marc Rosenbaum, a well-known energy consultant and now the director of engineering at the South Mountain Co., also likes them for three other reasons: the highly variable output made possible by inverter-driven direct-current components, the ease of zoning a house for heating and cooling, and the smaller drop-off in capacity with lower outdoor temperatures when compared to many conventional U.S. products.

He also adds a footnote to the cost discussion: the difference in cost between a 2-ton and 4-ton minisplit system is "significant," while the cost difference between a 80,000 Btu/hour furnace and a 120,000 Btu/hour furnace is not. That's one more compelling argument in favor of spending money for insulation, air-sealing and good quality windows.

David,
I suggest you send an e-mail to Customer Service at customerservice@greenbuildingadvisor.com with information about your problem. Feel free to contact me directly as well: Martin [at] greenbuildingadvisor [dot] com.

1.
Nov 9, 2012 4:04 PM ET

Information availability by David Prendergast

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